


an adaptive response

by Kate_Wisdom



Category: DC Extended Universe
Genre: First Time, Hurt/Comfort, Kryptonian Biology, M/M, Mating Cycles/In Heat, biology related breeding kink, bottom!Clark Kent, in heat!Clark Kent, omega!Clark Kent - Freeform, smut with hidden feelings, unwanted heat
Language: English
Status: Completed
Published: 2020-06-27
Updated: 2020-06-27
Packaged: 2021-03-04 05:40:11
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 5,616
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/24638476
Author URL: https://archiveofourown.org/users/Kate_Wisdom/pseuds/Kate_Wisdom
Summary: Superman's the last of his line. Biology has decided he needs a mate.
Relationships: Clark Kent/Bruce Wayne
Comments: 24
Kudos: 370
Collections: Heat Fic Summer 2020





	an adaptive response

**Author's Note:**

  * For [theae](https://archiveofourown.org/users/theae/gifts).



It had become an unexpectedly long night.

The evening had kicked off with a battle against a metahuman called Adam. Glittering with magic, intent on taking control of Metropolis, the sorcerer had incapacitated Superman in their fight in the skies above the city. It was only when Batman arrived on scene with the young champion Shazam that the League had finally managed to prevail.

In the aftermath of the battle, after federal agents had taken the sorcerer into custody and with Wonder Woman supervising the clean-up efforts, Superman realized that something had changed. His body temperature was elevated, his vascular pressure had increased by eleven per cent, and his heart had started to pound at forty beats a minute. This didn’t seem immediately life-threatening, but he was put on his guard all the same.

Maybe this was just a new facet of his powers? Since his resurrection, it seemed as if he discovered new abilities every day. 

The resurrection had also brought home to him the nature of Superman’s responsibilities. Growing up on the Kents’ farm, he’d managed to live in blissful ignorance of duty for so many years. Now, such naïveté was no longer possible in this new world of metahumans and supervillains, and heroes who were all that stood between innocents and the forces of destruction. 

It made him also realize he'd would never be one of them, despite everyone’s best efforts to the contrary. Yesterday, when Lois and Jimmy had thrown Clark Kent a thirty-ninth birthday party, he’d said how surprised and happy he was to celebrate with his friends, but that evening he’d gone home alone so that Superman could keep them all safe.

Superman turned his vision inward: to the internal organs that resembled those of his teammates, but which, thanks to untold millennia of divergent evolution, weren’t in fact alike at all. The sac over his heart that regulated his abilities was pulsing unusually, and the spiculated organ which he had instead of an excretory system was lit up like a Christmas tree. His endocrine glands, the equivalent of human adrenal glands, seemed to be producing a variety of new hormones which he could not immediately identify. And as if that wasn’t strange enough, further south, his reproductive system had come alive in a way that it had never done before.

Superman frowned. He had never felt less like Clark Kent than he did then, standing in the gathering dusk on a rooftop above his adopted city, unfamiliar chemicals charging through his body and something unmentionable occurring below the proverbial belt. 

Perhaps he was coming down with the Kryptonian equivalent of the common cold. But then, apart from his encounters with Kryptonite, Superman had never been unwell for a day in his life.

He found he was sweating profusely, the new hormones secreting an odor too faint to be detected without enhanced senses. This, too, was unusual: Superman had never needed to excrete fluid through his pores in order to adjust the temperature of his skin.

“Are you all right?” 

Superman steeled himself not to react. It ought to be impossible for anyone to sneak up on him, but somehow Batman managed it every time.

“What makes you think something’s wrong?” he hedged. He didn’t normally react this defensively, either, but this was the effect that the Batman had on people, including on his teammates and on the one person who would call Batman his friend. 

Batman stared at him impersonally through the circuitry of his mask, which would have revealed the same superficial physiological elevations in temperature and heart rate that Superman’s superhuman senses did. Instead of citing measurements to him, though, Batman asked, with uncharacteristic patience, “Was it that last spell? It looked like it really knocked you for a loop.”

Superman considered this. Adam’s magic might indeed have contributed to his present condition; Superman’s powers seemed to be almost as susceptible to magic as they were to Kryptonite. 

“If so, it might mean that it'll take magic to fix,” Batman offered. “Come back to the Batcave with me and we’ll run some tests. If whatever’s wrong with you requires a magical solution, I’ll fetch that Shazam kid again and we’ll do the necessary.”

Go to the Batcave for tests? Let Batman get close enough to inspect what was really going on with Superman’s alien biology? He didn’t understand why, but the thought made a shiver run down his spine, and Superman never felt the cold. 

Batman stepped forward expectantly. Superman could usually hear every human heart within a hundred mile radius, could smell the varieties of scent and fragrance of their sweat. Yet, for some reason, his sensory field had blocked out everything apart from this one man’s heartbeat, this man’s unique scent. 

“I’m fine,” Superman said, retreating from his teammate. “Everything’s fine. I’m on top of it.”

“You won’t be on top of it if it’s _magic_ ,” Batman called after him, but Superman was already miles away.

* * *

After his teammates brought him back from the dead, Superman and S.T.A.R Labs had relocated the Kryptonian scout ship from the government’s containment facility near Heroes Park to a secure bunker in the outskirts of Metropolis.

Now, in the early hours of the morning, Superman arrived at the ancient colonial vessel. He had spent the last few hours adrift in the night sky, trying to cool his restless flesh by sheer force of will. He’d hoped that doing his job might divert him from the strange not-quite-sickness that had seized him, but catching a jet-liner in freefall over the Indian Ocean and fetching a young boy’s kitten from a tree in Santa Monica had proved too-fleeting distractions.

He had known, sooner or later, that he would have to return to this place for answers: the only surviving link to his lost planet.

His presence on the bridge triggered the dormant systems to wakefulness.

 _Welcome, son of Jor-El,_ the ship’s computer greeted him, impersonally. Before the battle with Zod, the computer had approximated Jor-El's personality, the scientist who had bonded a generation’s worth of genetic information to his newborn son's cells and sent him off to a distant planet to preserve Krypton's legacy. Zod had wiped out Jor-El’s personality codes, and he’d done his best to destroy Jor-El's son as well, together with the Codex that Superman carried within him.

Superman said, hesitantly, “Computer, last night after an encounter with a metahuman magic user, I started feeling unwell. I need to know what’s wrong - - whether I’m really sick, or if it’s because of the magic.”

 _Scanning._ A beam of light enveloped his form. A brief second, and the computer announced, _Scans determine no disease or remnants of magic. You remain in optimal health._

“I don’t at all feel as if - -” He broke off, realizing he was asking the wrong question. “My temperature and heart rate and hormonal activity are all elevated. I wish to understand the extent and origin of these changes.”

The computer said, _As you are aware, my power levels are sub-optimal. For more accurate diagnostics, please activate the Mother Box, or enter the Genesis Chamber._

It seemed that all things led back to the Chamber. Superman stripped off his suit and entered the blood-red fluid of the pool.

He waded into the waters, bare as he’d been on the day he was born, his nude body looking to him as it always had, all muscles and smooth skin. He stopped when the Chamber’s fluids reached his neck, and looked up at the computer. “Try it now, please?” 

_Scanning_ , the computer announced, and a host of files shimmered into view before him - - encrypted schematics and scans and historical documents which the ship’s minimal systems strained to unpack. A diagram of his own inner organs: the massive heart, the multiple vessels of the lungs, the aorta looping in the abdomen and stretching out to his limbs, and a small, bud-like sac between his rectum and his stomach that had not been there the day before.

 _It seems your system is experiencing a sudden onset of somatic growth and adaptive reproductive maturation,_ came the impersonal diagnosis.

There was a ringing in his ears that didn’t appear to be due to tinnitus or any other external cause. A metallic taste flooded his mouth; a sickly-sweet fragrance filled his nostrils. 

“What? _How?_ ”

The computer repeated its analysis, as impenetrably as the first time, adding, _These archives have been compromised by Zod’s first attack, and the lack of power. But data suggests it may be because you have just reached a quarter of your natural lifespan, the time when Kryptonian Thinkers originally achieved sexual maturity.”_

“Wait one second," Superman spluttered. “I don’t know how this would have worked on Krypton, but my, my sexual equipment’s been working just fine for, I don’t know, the last twenty years!”

 _Just fine_ , of course, was an understatement. Before he learned better aim, teenaged Clark had torn holes in bedding and headboards, and on one memorable occasion, the south side of the Kents’ hayloft. Even these days, he had to be careful to come into the palm of his hand or the invulnerable stuff of his cape, and never too close to the few sex partners he’d had, so that he wouldn’t take unnecessary risks.

The computer hummed. _Your genitalia are merely one aspect of your sexual equipment. You are the last of the House of El, and also host to the Growth Codex. Now you have also achieved the age of adrenarche, your body appears to be experiencing a vestigial adaptive pubertal maturation which no Thinker has seen in generations.”_

“I don’t understand. Jor-El told me…” He remembered the hologram of his birth father as clearly as if it had been yesterday. “Jor-El told me that the Thinkers’ Guild, that _all_ Kryptonians, had evolved beyond sexual reproduction, that was why they had the Growth Codex and birthing chambers instead. If that’s true, then why is this adrenarche still happening?”

_Unclear. It is correct that, over the millennia, Kryptonians' genetic pre-engineering resulted in the sexual impulse being bred out of their populace. But you were not similarly engineered; instead, you are the first child naturally conceived by Thinker parents in centuries. Analysis suggests this is relevant._

Jor-El had mentioned this, too: that Jor-El and Lara committed a crime against the Krypton state, that their son's conception had been an act of rebellion as well as of love. But that still didn’t explain what was happening to him now. 

The computer had mentioned _one_ aspect of his equipment…? He stared harder at the image of the strange, alien sac, shimmering in the air above him, and, slowly, horribly, a realization began to dawn. 

“What, exactly, is _that_ …?”

“That’s what I’d like to know, too,” remarked a familiar voice, and this time Superman did jump out of his skin.

This happened _every time_ ; he didn’t know why he was always so surprised. He had to admit, though, that as far as surprises went, it was usually a welcome one. “Bruce. I won’t even ask you how you got in.”

“You forgot to put me on your list at the door, so I had to gatecrash?” Bruce Wayne, dressed in a designer suit, his billionaire’s spit-curl meticulously combed on his forehead, smiled his sardonic half-smile. “I know you weren’t going to bother asking me if I’d had you followed, either. But of course Batman’s going to want to know where Superman is, especially if he’s not feeling so hot.” He paused, frowning. “Speaking of which, Clark, you’re looking even worse than you looked last night. Also, do you smell something that’s not right?” 

Superman - - Clark - - couldn’t smell anything other than that pervasive, hormonal scent. He rubbed his own forehead, where he could feel pressure building. He couldn’t remember if he'd ever had a headache, let alone one that felt exactly like all medical descriptions of a frontal lobe migraine, but there was obviously a first time for everything. 

“About last night: I think we can rule out magic, at least.”

“Is this a Kryptonian biology thing, then? What does the computer say?” Bruce walked across the bridge to stare at the computer-projected image of Superman’s insides, which image shivered in the air and then abruptly winked out.

 _Insufficient power, shutting down now,_ the computer announced. _Please activate the Mother Box for optimal results. In the meantime, I have downloaded some relevant material into this readable device._

Bruce held out his hand for the device as it popped out of the nearby wall outlet, and Clark jolted out of the pool at superspeed to beat him to it.

A look of annoyance flared in Bruce’s eyes. Then those eyes flickered downwards, and widened, which was when Clark remembered he’d gotten undressed to enter the birthing chamber. 

His moment of excruciating embarrassment was interrupted by a wave of bone-shaking pain. 

It felt as if the sorcerer had cast a spell that had turned his insides out, as if Steppenwolf was ripping his spine from his body. Clark didn’t need to eat, hadn’t eaten since his birthday party, but he found himself collapsing to his knees and doing his best to spew his guts out.

“Clark? _Clark._ Talk to me.” Bruce was at his side, gripping him by the shoulders. His fingers were cold against Clark’s burning flesh.

As the pain and nausea receded, they left something equally all-consuming in their place.

Bruce was holding him in strong arms, vigor surging in his veins, powerful enough to fill Clark to the brim as he was suddenly craving to be filled… 

Clark pulled away, shivering in every limb, struggling for control. 

“Don’t touch me. I’m fine.” He was slow, _too slow_ ; his skin felt suffused with helpless fever. A miasma of scent was seeping from his glands through his pores and clogging the air between them. 

He clawed his way into his costume at half his usual speed, desperately covering himself up, when all he really wanted to do was tear his uniform from his shaking, too-hot body. 

Underneath his ribs, his new organ gave off little pulses of sick heat.

“No. Wait.” Bruce had gone uncharacteristically pale, looking almost as unwell as Clark himself felt. “I’m going to borrow or buy another Mother Box and get this ship powered back up so we can get some answers. We’ll figure this out together.” 

_So we can get some answers._ Even without the ship’s computer, Clark thought he now had a pretty good idea. Hard not to, really, while being battered by wave upon wave of what he could no longer deny was pure, ravenous lust. 

He had always thought of Bruce, of Batman, as the one person whom he didn’t have to protect from the villains who threatened their world. He hadn’t imagined that the one person he might need to protect Bruce from was from Superman himself.

He had to put some distance between them.

Clark forced himself to say, “No need. I can fight my own battles,” and then he was out into the open and streaking through the sky, trying to get far enough away.

* * *

Denali was the most prominent and most isolated peak in Northern America: a granitic massif in the Alaskan range, birthplace to glaciers and ice fields. When Clark got there, the meteorological station installed on a ridge near the summit – - the third highest weather station on Earth – - was recording a temperature of −70.5 °F.

Clark didn’t feel the cold at the best of times. He'd thought this extreme climate would somehow cool down the hormones that were now firing on all cylinders, and he'd soon discovered he was mistaken. Encasing himself in the ice of the Antarctic Polar Plateau, or flying into the ultimate frigid temperatures of space, wouldn’t have quenched this terrible fever.

The frosty mountain air washed over him, the crisp odor now turned nauseatingly sweet. His bulletproof skin felt as if it was burning up from the inside. All he wanted to do was lie down in the snow and give voice to the whine crawling up his throat, howling his need into the atmosphere until his lungs gave out at last.

Grimly, Clark pushed his suffering aside. Ma and Pa Kent hadn’t raised their son to be a whiner. Anyway, the idea wasn’t so much to treat his symptoms, wretched though they were, as to ensure he stayed far enough away from humans so he wouldn’t be a danger to them, or a liability to himself.

Judging from the cloying scent that surrounded him, his body was putting out enough pheromones to attract every red-blooded creature with a pulse and a functioning sex organ. He wasn’t certain how far the behavior-altering chemical field extended. Through the heart of a snowstorm, with not even animals around for hundreds of thousands of miles? Clark hoped he had gotten far enough away.

He told himself the unendurable heat couldn’t last forever. That it had to snuff itself out eventually, once Clark’s alien physiology realized Clark wasn’t going to propagate the line in the old-fashioned way. After a while, the genetic switch would flip off again, and generations of modern Kryptonian reproductive engineering would reassert itself, and let Superman get on with his duties and Clark with his normal life. 

Clark just needed to hold on until then, however long that would be. 

The scant data provided by the ship’s computer hadn’t answered that question. Unsurprising, really, since apparently the last time in Krypton’s history when this sort of reproductive compulsion had occurred had been in the bad old days when Krypton was still ruled by kings. 

That last Thinker king’s name had been Val-El. He'd birthed twelve children to his Warrior mate, and between them they’d managed to repopulate the Thinker line. Until the clone-hybrid wars, when the Guilds had elected to close the door to emperors as well as dangerous emotions, and created the Council of the Wise to govern them and birthing chambers to reproduce.

Clark didn’t know whether to envy this ancestor and his happy family, or to rue the evolution that had led to this vestigial second puberty. The way he felt now, it definitely was the latter. 

He’d never known what it was like to hurt like this, to be completely in the power of his biology. It had been long hours, and Clark had just gotten more and more aroused, as if he were slowly but surely bursting out of his uniform, or at least the lower part of it.

The sun began to dip below the horizon, hidden from the human eye behind the sheets of ice and snow. Soon it would be even colder, and truly night. 

Clark knew he had to pace himself, but as the minutes crawled by for him on the mountain, shivering and alone, he finally grew desperate enough to consider taking the edge off a little.

He leaned against an icy outcropping of rock, loosened his uniform, and cautiously took himself in hand.

Immediate pleasure rushed through his body. He couldn’t muffle the groan that burst from him. The sensation of contact, of skin against skin, after hours aching to be touched, of hurting and wanting and staunchly withholding; it felt so indulgent, so good. Panting, he stroked himself slowly, trying to make this last, but he knew he couldn’t hold on. At the last moment he remembered to curl his fingers as he came, shuddering, into his palm. 

He took a moment to catch his breath, savoring the feeling of relief. He wiped his hand absently against the ice, glad to see he’d managed to control himself enough not to claw a fist-sized chunk out of the mountainside. 

The moment of respite was short-lived. In the pit of his stomach, the fire started to kindle again, his balls beginning to tighten and his cock to fill with blood. Clark started to pull his uniform back up before he realized there was more liquid, fragrant and slippery and very different from his normal spunk, dripping between his asscheeks onto his thighs. 

Cautiously, Clark prodded himself back there, and then he came again, violently, crying out in surprise. 

This time, his ejaculate ripped a small slab off a nearby boulder. 

Clark collapsed to his knees, gasping. This was not good news at all: indulging himself had somehow made him even more sensitive. His heart was pounding, his cock felt swollen and tight and even harder than ever, and his hole was dribbling even more of the sweet-smelling slick that blatantly announced what was clearly his first heat. 

The snow hurtled down, the gusts of wind on the summit making it almost impossible for anything human to breathe or to survive. Clark rested his burning forehead against the frozen ground and let himself close his eyes for a moment. If only there was some comfort, some friend or companion to help him through this ordeal like Val-El had long ago. 

It was impossible, of course. Superman had no one whom he could afford to endanger, only himself. 

It was going to be a very long night.

At first, when the faint whirring noise approached, he chalked it up to his imagination. But there was no imagining the sound of that familiar heartbeat, or that voice, muffled by a ventilator, calling his name.

Clark opened his eyes as the shadow fell over him, armored boots crunching in the snow.

“You’re a hard man to track down,” Batman said, mildly. He was wearing a quarter ton of equipment and a jet-pack that was doing nothing for their remote, pristine surroundings. 

Despite himself, Clark’s heart leaped. He had never seen a more welcome sight than his friend, here on this mountain, coming to his rescue. Weakened by biology and longing, he found himself shamefully close to tears.

Batman knelt in the snow, maneuvering out of his gear with some difficulty. He lifted the ventilator from lips that were chapped and pale with cold. 

“Before you fly away again, let me tell you I didn’t come to fight your battles. I didn’t come to fight at all, really. What I came for …”

He took hold of Clark by the biceps, twisted him into his arms, and kissed him.

It was probably one of the least romantic kisses billionaire playboy Bruce Wayne had ever given, but Clark was in no position to criticize. Thinker synapses flared through his body, the Codex bonded to his cells roared to life, and every biological impulse within Clark strained toward the sanctuary Bruce was offering.

It took all the control Clark had to pull away, to say, thickly, “You shouldn’t have come.” 

At this altitude and in this wind, and also at the mercy of Clark’s pheromones, Bruce struggled for breath; yet, somehow, he was smiling wryly. “Why did I think you might say that? I know what’s happening to you, Clark. You can’t ride this out alone.” He paused before adding, “Also, if you flew off and left me here, I might asphyxiate before I froze to death.” 

Clark smiled back, despite himself. “Is the jet at Base Camp?” 

“Are you asking if I parked my Batplane on the actual glacier? You’ll be pleased to know I’m not _that_ much of an asshole. Isn’t there a research station further down the mountain?”

* * *

The International Arctic Research station was indeed at an altitude to support reasonable oxygen saturation levels, at least in a man of Batman’s enhanced fitness. Nevertheless, Clark made sure he collected Batman’s supplementary oxygen tank as well as the solar-powered heating device, and set about making its cramped interior comfortable enough for his friend.

Bruce took his cowl off to watch. His eyes glinted with amusement in the battery-powered lamplight. 

“You don’t have to do whatever this nesting thing is on my account,” he drawled at last. “I told you, I’m here to help.”

What Clark wanted to do was to leave Batman here in this safe place while Superman purged his terrible clawing fever in the heart of the sun itself. Instead, he made himself meet his friend’s eyes. He owed Bruce the truth. 

“You can’t help with _this_. No one can. Centuries ago, it seems the heirs of El could develop both sets of sex organs, to fully maximize reproductive potential. I’m the last of the line, and I’ve just hit puberty.”

“I know,” Bruce said, neutrally. “Before your scout ship turfed me out this morning, I scraped the last few megs of its download.”

This stopped Clark in his tracks. “You _know_ \- -?” Bruce knew that Clark had just gone into his first heat like his ancestors had long ago, that he'd been driven to mate and to reproduce? “And still you came here?” 

Clark’s new reproductive organ thudded underneath his uniform, a rhythm that could no longer be refused.

“Of course I did.” Bruce crossed the small room and put his arms around Clark’s neck. “Look, what did your ancestors do to bring their heats to an end?”

Clark swallowed. Bruce’s nearness was making him dangerously unsteady; the scent rising off Bruce’s skin was irresistible. He fought to keep his voice even. “There were suppressants, but you needed to take them before the onset of each cycle. Otherwise, you ended the heat in the old-fashioned way - - by having sex with your mate, and hopefully making a child to carry on the line.” 

He took a deep breath. “But I don’t have a mate. And even if I did, I couldn’t give any human a child of my line.”

Even as Clark said this, he knew that wasn’t what his biology required. In the circle of his friend’s arms, his hole was soaking through his uniform, his empty womb aching viscerally to be filled.

Bruce smiled faintly in understanding. “But you do have a mate, Clark. Let me be that for you now, at least.” 

He put his nose into the join of Clark’s throat and jawline, at the place where the seam of his uniform began, and inhaled deeply. Clark didn't know what Bruce's offer meant - - whether he was just saying it to help Clark, or if he was being serious - - but felt his defenses unravel, one by one, his muscles working slowly loose, surrendering to Bruce’s touch. He leaned against Bruce’s solid body and closed his eyes, and let himself set his burdens down at last. 

Then Bruce murmured, “Let me give you _my_ child,” and bit down, hard enough to mark skin that wasn’t Kryptonian, an unmistakable sign of just how serious he was, and a humiliatingly needy noise broke free from Clark’s throat. 

“Bruce - - oh, God - -”

“It’s all right,” Bruce said, as Clark began to grapple with his costume with clumsy fingers; “let me,” and between them they managed to wrestle off first one uniform and then the other, until they were both finally naked with each other. 

There was a pallet in the corner of the room. Bruce spread Superman’s cape across the mattress, slung Clark on top of it, and followed suit, pulling Batman’s cloak over both of them.

It was hard to think from within the white heart of his heat; impossible, with Bruce lying flush against him, all taut muscle and bare gooseflesh, hard cock jutting against Clark’s hip. Clark’s mind was an urgent mantra of want, and _good_ , and need; helplessly, he rutted against Bruce’s body and tried to hang on to the remnants of his self-control. The last thing he wanted to do was to hurt Bruce in his eagerness, now that he had finally lowered his guard and let in the one person who understood just how lonely he was.

Bruce had let his own guard down, too; he pinned Clark to the mattress and trailed biting kisses against Clark’s exposed throat. “You smell great,” he muttered, “clearly this is one more Kryptonian sex thing, designed to make all potential mates crazy. God knows it’s really working.”

“Score one for the geneticists,” Clark managed. He knew how perilously close he was to Bruce's vital organs, his cock tight and dripping where it pressed between their bodies, the slick soaking out of him, but he wanted Bruce so much it was unbearable. “Now, please, I need - -” 

He couldn’t say it, lifted his hips wordlessly instead, and Bruce ran a calming hand along his flank. 

“Yeah, all right, I know what you need. Up you get.” He shifted his weight off Clark, and then flipped him over so Clark was face down on the pallet. Clark panted into the material of his cloak as Bruce’s almost-gentle fingers began to explore the ridge of muscle around Clark’s swollen hole. 

Clark had no idea if Bruce had ever been with a man; Bruce had certainly never been with an alien from Krypton. At another time, Clark might have wondered whether Bruce found his strange anatomy unappealing. Now, though, naked and burning from his scalp to his heels, he had moved beyond shyness, beyond anything other than a bone-deep craving for Bruce to take him, to fuck him with his fingers and then with his cock. 

He heard whining noises, vaguely registered he was the one who was making them. Bruce held him down with an open palm between his shoulderblades as he worked his fingers into Clark’s hole. “Damn, you’re wet,” he muttered, more to himself than to Clark. “I didn’t need to load up on lube after all, this is going to be a walk in the park.” 

Clark wasn’t so sure about that, but then the Batman was always full of self-confidence. Bruce readied himself, took hold of Clark’s hips and, with an effortful thrust, finally slid himself home.

Clark heard himself cry out, and then he couldn’t form words anymore. The lineage of his ancestors reduced him to little more than a rutting animal in heat, driven by the blind desire to be mated. Helpless in the grip of biology, he found himself mindlessly lifting his ass and spreading himself for Bruce and begging urgently to be bred. He’d been alone on the mountain, hurting, choking with need, and finally his mate was here: hands holding him down, hard cock filling him up - - please, _please_ \- - driving savagely into his body, and taking possession of him at last. 

Bruce held him and panted into the nape of his neck, closer and more intimate in this moment than Clark had ever allowed anyone to come, closer than maybe Bruce had ever been with anyone himself. “I can’t believe how good you feel. Everything about this, everything about you, Clark, you’re _mine_ , you hear me? You belong to me - - ”

Bruce couldn’t finish, either; he broke off, his teeth fastened helplessly into Clark’s shoulder, struggling to hold himself back and failing as he took Clark face-down in a searing rhythm that obliterated all words and all control.

The ancient mating claim laid hold of him, and his mate balls-deep inside him, and wrecked them both in a gasping, writhing, white-hot blaze of light.

Clark jerked back to himself in seconds. He was _slow_ , too slow; his climax seemed to have turned his arms and legs to water; he had to fight Superman's learned instinct to get as far away as possible. 

This time, though, his mate wasn’t in any danger. 

Bruce had collapsed on top of him. He was panting hard, trying to catch his breath at this altitude, murmuring something into his hair that sounded like, “You’re mine, damn it.” 

“Yeah, yours,” Clark agreed. Cautiously, he let himself relax in Bruce's arms, basking in the aftermath of pleasure, shifting his comfortably sore limbs beneath Bruce’s weight. 

Which was when he felt the flare of a large engorgement stopping up his hole.

“What in the world - -?”

“It wasn’t hard to figure out,” Bruce said, sounding amused as well as breathless. “Your cock has this swollen gland at its base that humans’ don’t. It’s not an uncommon reproductive tactic to knot a mate during sex, like wolves and dogs do… It took some doing, but I engineered a condom with a knot that inflates.”

Astonished, Clark turned to look at Bruce. “You weren’t kidding, you did really come prepared.”

Bruce had pushed himself up on his forearms; he’d gone red in the face, but was looking rather pleased with himself. “Yeah, I loaded for bear before I set out. It’s not every day you get the chance with Superman.” He paused, and then brushed his knuckles against Clark’s cheek. “Or the chance with Clark Kent, either.” 

Clark had to swallow. Dangerous emotion rose inside him, and he didn’t think it was just because of the hormones. “You know how I feel about you, Bruce.”

“Now _you’re_ kidding. As far as Batman’s concerned, Superman's the biggest cock-tease on the planet.” Bruce was smirking, though, and Clark had to try to wipe that self-satisfied grin off his mouth by kissing it soundly. 

"Well, that's not true anymore, is it?” 

Maybe Bruce wasn't being serious; maybe this was just for one night. But, call him old-fashioned, Clark didn’t think Bruce Wayne had ever put in this much effort for just a one-night stand. As Clark, as Superman - - he found he was ready to surrender everything he was to this friend, this person who could share his burdens. This mate.

Who, it had to be said, seemed more than willing to shoulder those burdens, as he reached around for Clark with unmistakable intent. “How long before you need to go again? Just you wait and see what other tricks I’ve packed for you.”

Clark couldn’t wait. It was going to be a very long night, and he intended to savor every moment of it.

**Author's Note:**

> Most grateful to Kai and Morbane for betaing.
> 
> [ Physiology per Superman 1, Issue 656 (Nov 2006), penciler Carlos Pacheco](https://p.dreamwidth.org/5cce10baedec/-/politedissent.com/images/oct06/sman_656.jpg).  
> [Canon](https://krypton.fandom.com/wiki/Kryptonians) [Kryptonian reproduction](https://dccomicsextendeduniverse.fandom.com/wiki/Kryptonian).


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